Humans are endothermic: we produce our own body heat so that our bodies can stay warmer than the surrounding environment. So when you go swimming in 80° water you are still able to keep your body temperature warmer than the water, allowing your body to function normally. All birds and mammals are endothermic. Other animals like reptiles and fishes, however, are ectothermic- or cold blooded. This means that the temperature of their external environment regulates their body temperatures. That is why you commonly see snakes and lizards laying on sunny rocks in the morning: they need to warm up before their bodies are able to function at peak levels.

Fishes’ body temperatures are strongly regulated by the water temperature. Fishes use their gills to absorb oxygen dissolved in the water. This allows them to stay underwater their entire lives, but also comes at a cost. Because the blood vessels in the gills are in such close contact with the water, body temperatures are very susceptible to cooling down to the temperature of the surrounding water. As a result, fishes living in cold water have a body temperature very close to that of the surrounding water, and therefore, have decreased performance, a slower metabolism, swim slower, and overall are not as active as fishes living in warmer water. (For comparison sake: our lungs are located at the center of our body- far away from surfaces which come in contact with the external environment. When we breath cold air it warms up after entering our nose to a temperature that does not cause our blood to cool significantly when the oxygen is absorbed into our bloodstream.)

Some fishes, like tunas and some sharks, are capable of warming certain parts of their bodies (i.e. brains, eyes, or muscles used for short bursts of speeds) while in colder water. These fishes are termed “regional endotherms” and have higher performance levels while hunting, especially while diving to deeper water (water below about 100 m can approach near freezing temperatures!). For example, tunas and mackerels can warm their eyes and brains, increasing vision and nervous response while hunting in colder waters and mako sharks can heat muscles allowing them to swim faster for short time periods. This alone is pretty impressive in the fishy world. Scientists have discovered something even more exciting!

Ladies and gentlemen, please put your hands together for the opah!!

The opah, or moonfish (Lampris guttatus) is an extremely bizarre looking fish (Fig. 1). This disc-shaped, silvery spotted fish with bright red fins looks like something out of a children’s book. And this fish gets big (Fig 2)- reaching lengths over 6 feet long and weights over 150 lbs! While the opah can be found in all temperate and tropical oceans, not much is known about this strange fish. It is apparently solitary and spends most of its time hanging out in deep water (50-500 m).

Figure 2: Three happy men with their large, freshly caught opahs! Image from masterok.livejournal.com

Until recently, scientists thought that this fish was lethargic, like most deep-dwelling fishes. That is until some scientists from NOAA took a closer look at a specimen they had just caught. When they looked at the gills, scientists were very surprised to see a unique network of blood vessels that appeared to be capable of warming up the tissue. This system, termed the rete mirabile (latin for “wonderful nets”), uses countercurrent heat exchange to warm the opah’s body. Essentially, the blood vessels which are cooled by the water are interwoven with blood vessels carrying warm blood from the body. The warm blood vessels, warm up the cool blood vessels, keeping the body warm even in cold water(Fig. 3)!

A Closer Look:

After the initial discovery, the NOAA scientists decided to dive a little deeper. They measured temperatures throughout the body of recently caught fish and found that, unlike regional endotherms, the entire body of the opah was warmer than the surrounding water (5° C warmer on average; Fig. 4)! In situ measurements confirmed that the pectoral fin muscle generates the bulk of this heat. Layers of blubber surrounding the gills, heart, and pectoral muscles help to insulate the fish and maintain the higher temperature.

Figure 4: A) The internal temperature of the opah is higher than the surrounding environment (ambient temperature of 10.5°C). Temperatures are warmest around the brain (red area), gills, and pectoral fin (directly behind the red area- yellow to light blue in color) B) In situ measurements of body temperature at depth. Note the very stable internal temperature of the opah, even though there is a significant drop in water temperature.

Figure 3: Close up of the gills of an opah. A) shows a single gill arch. The close up in B) shows the thick blubber (adipose tissue) surrounding the gills. C) shows the rete mirabile with interwoven cold blood vessels (red – oxygenated) and warm blood vessels (blue – deoxygenated)

Using satellite tags, the scientists found that opahs spend the majority of their time in cooler water (between 50 – 500m) without spending much time in shallow water warming up. The opah is able to spend more time in deeper water and dive deeper than regional endotherms (such as tuna; Fig 5) because of this whole-body endothermy which increases performance and keeps the fish from having heart failure at such cold temperatures.

Figure 5: Comparison of the percentage of time spent at depth for the opah and the albacore tuna. The opah spends more time at depth and makes deeper dive than the tuna.

Significance:

With the increased body temperature, the opah is expected to have a higher metabolism and increased performance of muscles, eyes, brain, and heart- allowing for sustained swimming speeds, higher sensory capabilities, and lower risk of having cardiac failure while at depth. All of this contradicts the initial beliefs that the opah is some slow, goofy fish that just hangs out at depth waiting for its next meal to swim by.

Besides the super cool physiological significance, this recent discovery highlights another important point in science: there are a lot of really incredible things in nature that we have yet to unveil! We know astonishingly little about our world and sometimes, the biggest discoveries come from unexpected place and funny looking fishes!

I received my Master’s degree from the University of Rhode Island where I studied the sensory biology of deep-sea fishes. I am now working with the South Carolina DNR at Waddell Mariculture Center, working on enhancing stocks of marine fishes. I am fascinated by the amazing animals living in our oceans and love exploring their habitats in any way I can.